What is the most important hormone causing an increase in serum ca2+?

The answer: Parathyroid hormone (PTH)

Parathyroid hormone (PTH, parathormone, parathyrin) is secreted by the parathyroid cell of the parathyroid gland as a polypeptide containing 84 amino acids. It works by increasing the concentration of calcium (Ca2 + ) in the blood, while calcitonin (the hormone-producing the parafollicular cells (C cells) of the thyroid gland) lowers the calcium levels.